A gas concentration measurement system based on wavelength modulation spectroscopy (WMS) technology is the main tool for ammonia escape monitoring in coal-fired power plants. In this paper, we designed a numerical simulation experiment to optimize the modulation parameters of NH3 molecular absorption spectroscopy, and it was found that the optimal modulation depth of WMS measurement system is 0.238 8 cm-1 under the condition of 200 ℃. In the temperature range of -20~500 ℃, the optimal modulation depth decreases nearly linearly with the increase of temperature, and the relationship between the optimal modulation depth, am, and the Celsius temperature, T, can be precisely described by the equation am=0.291 8-3.265 14×10-4T+2.282 14×10-7T2. These results are of great significance for the optimization of the modulation parameters of the ammonia escape monitoring system in coal-fired power plants.